1. Field of the Invention
This invention is related to a lymph-like fluid composition and a method of using the composition to protect the brain and spinal cord during resuscitation of cardiac arrest.
2. Background Information
It is estimated that more than 350,000 Americans died of sudden cardiac arrest each year, more than 95 percent of victims die before reaching the hospital. Economic costs for trauma related arrest is over 400 billion dollars each year. Despite numerous scientific advances throughout modern medicine, outcome of resuscitation for arrest victims remains poor. The cardiopulmonary resuscitation (CPR) practice including ventilation, closed chest compressions, open chest cardiac massage and defibrillation (step ABC, i.e. Airway, Breathing and Circulation) was established in the 1960's. This CPR protocol has not taken vulnerability of central nervous system (CNS) into account. Therefore, the key obstacle for current resuscitation is the acutely limited time window, and the major reason for the limited time window is that the CNS is extremely vulnerable to hypoxic-ischemic insult. Traditionally, it is believed that the maximum tolerant survival time for brain in a cardiac arrest patient is about 5 to 8 minutes. Therefore, in clinic, the real problem in circulatory arrest is usually not to restore cardiopulmonary function but instead to prevent brain death.
Shock results in low blood perfusion throughout body. Although the blood perfusion is not completely stopped, shock shares many similar pathological processes with cardiac arrest, and is also a life-threatening condition. Shock can be categorized into anaphylactic, septic, cardiogenic, hypovolemic shock depending on the causes.
The CNS including brain and spinal cord is extremely susceptible to hypoxic-ischemic insults compared with peripheral organ systems such as the liver, kidney, lung, or intestines. The mechanism underlying this susceptibility has not been completely understood. Lacking of an effective approach to protect brain and spinal cord is the ultimate reason why the time window for resuscitation is so limited.
In peripheral tissues, capillaries are relatively permeable and as a result the interstitial fluid (ISF) contains about 2 g/dl of plasma proteins. It is believed that interstitial proteins and hyaluronic acids form a dense network of proteoglycan filaments so that the ISF moves molecule by molecule from one place to another by kinetic motion among proteoglycan filaments in the interstitium. Normally the amount of free-flowing fluid, present in the interstitium is small. A low interstitial protein concentration results in an increased amount of free ISF. An elevated concentration of interstitial protein may reduce the free ISF, but it also attracts more fluid, resulting in increased volume. The lymphatic system is the scavenging pathway for interstitial proteins. By regulating the removal of excess protein, the lymphatic system keeps the interstitial protein concentration around 2 g/dl. This ensures limited free fluid and also regulates the ISF volume. Lymph flow reduces ISF volume resulting in negative interstitial pressure. Therefore, the movement of proteins from plasma to ISF and finally to lymph is important for maintaining extracellular homeostasis.
The CNS lacks a lymphatic system; instead it is bathed by the cerebrospinal fluid (CSF). The CSF is very different from the lymph in peripheral tissues in at least two aspects: protein concentration and the resultant interstitial fluid pressure. The CSF is secreted by the choroid plexuses that line the cerebral ventricles. Tight junctions linking the adjacent choroidal epithelial cells form the blood-CSF barrier and prevent most large molecules from passing into the CSF from the blood. Therefore the CSF contains an extremely low protein concentration. In a human adult, the CSF occupies about 10 percent of the intra-cranial and intra-spinal volume. The average rate of CSF formation is about 21 to 22 ml/hr, or approximately 500 ml/day. The CSF formation is related to intracranial pressure (ICP). When the intracranial pressure is below about 70 mm H20, the CSF is not absorbed, and production increases. Many agents are known as CSF production inhibitors such as Furosemide and Acetazolamide. The choroid plexuses may not be the only sites for CSF production. Milhorat reported that in monkeys with choroid plexuses removed, up to 60% of the CSF is produced from ISF flow out of the brain. The blood-brain barrier (BBB) prevents proteins from entering the interstitium. Therefore, it is speculated that the ISF in brain, just like the CSF, has a low protein concentration. Importantly, the CSF is contiguous with the ISF, with the Virchow-Robin spaces, serving as a conduit. It is estimated that intracellular protein concentration averages about 16 g/dl in mammalian cells. Therefore water and Na+ in the ISF tend to move easily into cells. To make matters worse, the ICP averages about 10 mmHg leading to a positive interstitial fluid pressure. Taken together, these factors make the CNS prone to edema formation. As a result cells in the CNS constantly consume more energy to remove excess intracellular fluid in physiological condition. When cell energy is compromised, such as in ischemia following cardiac arrest, cells rapidly become swollen, i.e. cytotoxic edema.
Swelling of cerebral tissue can compress blood vessels inside the Virchow-Robin space leading to a persistent deficit in blood perfusion even after the restoration of blood perfusion, termed a ‘no-reflow’ or ‘low reflow’ phenomenon. This blood perfusion deficit blocks collateral circulation and induces a feedback loop contributing irreversible cerebral cell death and tissue necrosis.
The treatment disclosed in this invention to protect bran and spinal cord is based on the following measures: (1) reducing interstitial pressure in the CNS, and (2) increasing the concentration of water and ion-binding Polypeptides in the CSF.
Lowering the ICP reduces the interstitial pressure of CNS. For example, the CSF drainage to lower the ICP has been used to prevent spinal cord damage caused by cross-clamping aorta during aortic surgery for more than 50 years. Although it is beneficial in most of the cases, the clinical outcomes of this approach, have been inconsistent. This inconsistent result is likely caused by the CSF remained in the folds and chambers of the CNS after general CSF removal. The brain and spinal cord have complex contours with many sulci, gyri and pools. These complicated structures make it impossible to remove the CSF completely even when ICP is reduced to 0 mmHg. Moreover, surface tension and capillary forces retain CSF in the Virchow-Robin space and in the spaces between the dura and brain surface. This invention addresses problem of treating the remaining CSF after general CSF removal.
Researchers have suggested that bolus infusion of hyperoncotic solution into the cerebral vasculature or perfusion of hyperoncotic artificial CSF can alleviate cerebral edema. The term “hyperonconic” refers to high colloid osmotic pressure caused by the existence of large molecular weight substances that do not pass readily across capillary walls. For example, U.S. Pat. No. 6,500,809 to Frazer Glenn discloses a method of treating neural tissue edema using hyperoncotic artificial CSF. Several colloid osmotic agents including albumin and dextran were used in the method.
This invention, however, reveals that the colloid osmotic pressure is not a key factor. Although albumin is effective in protecting the CNS tissue, it appears that its colloid osmotic effect is not the primary reason for its neural protective effect, because other colloid osmotic agents such as Dextran and Hetastarch are ineffective. In contrast, gelatins, even with molecular weights smaller than cut-off size for colloid osmotic agents are effective. In fact, gelatins with various molecular weights ranging from 20,000 to 100,000 Daltons are all effective regardless of their molecular weights. Collagen and Sericin peptides are also effective. Albumin, gelatin, collagen, and Sericin peptides all belong to poly amino acids category. It is thus the water and ions binding properties of proteins or other polyaminoacids that really matter.
The CNS can be made as resistant to various insults as other organ systems, or at least less vulnerable to such insults, by mimicking lymphatic system of other organs. The present invention is also directed at other mechanisms of ischemic injury that are common to all organ systems, including the use of insulin, magnesium and ATP.
The CSF contains about one fifteenth of plasma insulin concentration (CSF: 0-4 μU/ml; fasting plasma: 20-30 μU/ml). Insulin has also been regarded as a growth factor, evidences have repeatedly proven that insulin yield protection for ischemic cerebral tissue independent of its glucose lowering effect. Compared with other growth factors, insulin has been used in clinic for years, and is much less expensive.
Magnesium (Mg2+) is the second highest electrolyte intracellularly (58 mEq/L). ATP (Adenosine 5′-triphosphate) is always present as a magnesium: ATP complex. Mg2+ basically provides stability to ATP. At least more than 260 to 300 enzymes have been found to require Mg2+ for activation. Best known among these are the enzymes involved in phosphorylations and dephosphorylations: ATPases. phosphatases, and kinases for glycolytic pathway and krebs cycles. At the level of the cell membrane Mg2+ is needed for cytoskeletal integrity, the insertion of protein into membranes, the maintenance of bilayer fluidity, binding of intracellular messengers to the membrane, regulation of intracellular Ca2+ release by inositol triphosphate etc. Mg2+ also affects the activities of pumps and channels regulating ion traffic across the cell membrane. The potential changes in tissue Mg2+ might also affect the tissue ATP levels. In tissue culture and animal models elevated Mg2+ concentration has been repeatedly proven to protect neurons and other cells.
The concentration of ATP inside cells is high, whereas the concentration outside cells is very low. Harkness and coworkers showed that the ATP concentrations is about 1 to 20 μmol/l in plasma, however in CSF, ATP could not be detected, and it was estimated to be about less than 0.05 μmol/l. Mufioz and coworkers detected that the ATP concentration in CSF is about 16 nM/l. Exogenous ATP provides direct energy to the damaged tissue. Sakama and coworkers showed that continuous application of ATP (100 μM) significantly increased axonal transport of membrane-bound organelles in anterograde and retrograde directions in cultured neurons. Uridine 5′-triphosphate produced an effect similar to ATP. Mg-ATP has been used clinically in Japan to treat hepatic and kidney hypoxia-ischemia.
Acidosis is a universal response of tissue to ischemia. In the brain, severe acidosis has been linked to worsening of cerebral infarction. Recent evidence however suggests that mild extracellular acidosis protects the brain. It has been reported mild acidosis provide cell protection down to pH 6.2. The acidosis that accompanies ischemia is an important endogenous protective mechanism. Correction of acidosis seems to trigger the injury. It has also been speculated that mild acidosis might stimulate anaerobic glycolysis that might supplement NADH oxidation and ATP yields.